[ExI] Field trip

[Keith Henson]

Yesterday, for reasons too complicated to explain, I went on a field
trip to the General  Atomics Aeronautical  Systems  plant near San
Diego where they make the Predators.  After that, we went down the
road a few miles to the plant where another division of GA is making
the vertical solenoid magnets for ITER.

The Predator plant was mildly interesting.  They are making 4
variations on a theme.  Some versions can stay up for almost two days.
They also build the ground control stations in several different
versions. They didn’t think much of my asking if they were going to
make a private plane version.   They have about 6000 people working
there.

We then got a visit to the plant where GA is building the ITER magnet,
the one that goes into the center of the thing.

http://www.ga.com/central-solenoid-photo-gallery

Now that was an interesting tour!  The magnet plant was amazing

 The conference room had a whole wall with a one to one cross section
of the magnet.  It is wound with 2 inch square superconducting wire
that's made in 1 km chunks in japan.  It’s lighter than railroad rail,
but not much.

The stainless steel outer layer encases the stranded conductors the
become NbSn2.  It is pulled into an inner hole in the stainless steel
jacket that is about an inch in diameter.  This is done in Japan.  In
spite of all the troubles that have happened in Japan over the last
few years, GA has enough of this “cable” on hand for the first of 7
magnet coils they are making.

 The building they are in had been used for something else.  They
started by ripping out the old floor and replacing it with one two
feet thick, full of thick rebar, and extremely flat, something around
1/16th inch in ten feet.  That way they can move the coils around with
an air float vehicle.   There is a detailed description of the coils
here http://www.ga.com/ga-iter-energy-program

They were very slowly winding a test coil while we were there.  It
essentially amounts to straightening and then to bending two inch bar
stock into a very tightly defined circle and stepping from layer to
layer.  It takes several segments to make the whole thing, so they
have to splice it as they make the coils, then weld a replacement
segment of steel around the splice.  I didn’t manage to understand how
they were going to do this while keeping the temperature of the
material that will become a superconductor to less than 200 deg C.

After they wind it, they heat treat the coil for 200 hours at 650 C
(1200 F) in an oven to get the niobium and tin to combine.  After
that, they have to be extremely careful not to bend it much, but they
have to insulate the conductor with spiral wound fiber glass.  They
have a huge machine that lowers one turn at a time so the insulation
wrapper can put on the glass.  Then they soak it in epoxy.

Having the magnet quench is a serious worry because that can do
serious damage to the wires.  At 45 kA, opening the magnet into a one
ohm resistor would develop 45 kV initially, and that’s too much for
the insulation.  The initial dissipation would be 2 GW; with a half
ohm resistor, 1 GW.

I have an appreciation of big inductive kicks.  Many decades ago the U
of Arizona physics lab for lecture demonstrations had a huge toroid
inductor, maybe 50 pounds of iron and copper.  You could stick an ohm
meter across it and read around 30 ohms for half a minute before the
core saturated and the resistance fell to a couple of ohms.  I once
got knocked on my ass by breaking the meter connection while still
holding the terminals.

I don’t think much of hot fusion as a way to cope with humanity’s
energy problems, but I was sure impressed with the engineering and big
machines needed to make these magnets.  The work force for building
the magnets is only 35 people.